With the recent realization that petroleum supplies are not inexhaustable and that increasing use of fossil fuel supplies, such as coal, will cause undesirably high pollution levels, man has been searching for alternative forms of energy and clean fuels. Hydrogengas is one solution to these problems. The only biproduct of the burning of hydrogen gas with oxygen is water vapor. Therefore, man has been experimenting with ways to produce the vast quantities of hydrogen and oxygen necessary to supply fuel and energy to industry.
Water photolysis is the light catalyzed decomposition of water into hydrogen and oxygen gases. The net photochemical reaction for the decomposition of water can be illustrated by the following reaction: EQU H.sub.2 O(l) + light .fwdarw. 1/2 O.sub.2 .uparw. + H.sub.2 .uparw.(1)
the hydrogen generated by the above reaction can be burned as fuel or sold for use in numerous chemical applications. The generated oxygen can be used to support combustion or collected and also sold for use in numerous private and industrial applications.
The evolution of hydrogen and oxygen from a water photolysis apparatus is known in the art. A typical prior art water photolysis apparatus may include a titanium dioxide (TiO.sub.2) anode and a platinum cathode immersed in an electrolyte solution. Other prior art water photolysis apparatus are described in U.S. Pat. Nos. 3,925,212 and 2,011,149 issued to Tchernev and Nozik respectively. More specifically, the Tchernev and Nozik references utilize semiconductors for the anode and cathode of a water photolysis apparatus.
The absorption of solar radiation by an n-type semiconductor anode excites the anode material, creating electron-hole pairs according to the following equation: EQU semiconductor anode + solar radiation .fwdarw. e.sup.- + p.sup.+( 2)
Holes (p.sup.+) are the unoccupied energy levels of the semiconductor anode material when excited by solar radiation. The electrons and holes cause electrochemical reaction to occur at the semiconductor/electrolyte solution and cathode/electrolyte solution interfaces according to Schottky barrier theory. More specifically, the generated holes are attracted to the Schottky barrier at the semiconductor anode/solution interface, resulting in the combination of the positively charged holes and hydroxyl anions (OH.sup.-) in solution to form oxygen and water. This reaction is illustrated by the following equation: EQU 40H.sup.- + 4p.sup.+ .fwdarw. O.sub.2 .uparw. + 2H.sub.2 O (3)
wherein p.sup.+ is a hole.
The electrons generated from the absorption of solar radiation by the n-type semiconductor anode (see equation 2) flow to the cathode through a suitable connection, e.g., a wire. The electrons, flowing to the cathode, are then attracted to the solution and leave the cathode at the cathode/solution interface and react with water to form hydrogen gas and hydroxyl ions as evidenced by the following equation: EQU 2H.sub.2 O + 2e.sup.- .fwdarw. H.sub.2 .uparw. + 2OH.sup.- ( 4)
wherein e.sup.- is an electron. Thus, hydrogen forming at the cathode and oxygen forming at the semiconductor anode are the products of the electrochemical reactions taking place at the anode/solution interface and the cathode/solution interface respectively.
If a material is selected for the cathode with a high overvoltage for hydrogen, then a biasing voltage, sufficient to overcome the overvoltage, will have to be applied to the water photolysis apparatus at the cathode to drive the hydrogen gas-generating reaction. Overvoltage is defined as the excess voltage above the normal reversible electrode potential of a metal electrode required to decompose a solution or cause a decomposition on the electrode. Ideally, cathode materials with a 0.00 volt hydrogen overvoltage would maximize the generation of hydrogen gas or a given amount of solar radiation impinging upon an n-type semiconductor anode.
The use of TiO.sub.2 as a semiconductor anode in a water photolysis apparatus has been shown in the prior art. See, T. Ohnishi et al., Berichte Bunsen-Gesellschaft BD. 79, Nr. 6, pp. 523-525 (1975); Fujishima and Honda, Nature, Vol. 238, pp. 37-38, July 7, 1972; Laser et al., J. Electrochem. Soc. Vol. 123, No. 7, pp. 1027-1030 (1976); Mollers et al., J. Electrochem. Soc., Vol. 121, No. 9, pp. 1160-1167 (1974); Hardee et al., J. Electrochem. Soc., Vol. 122, No. 6, pp. 739-742 (1975); Fujishima et al., J. Electrochem. Soc., Vol. 122, No. 11, pp. 1487-1489 (1975); S. N. Paleocrassas, Solar Energy, Vol. 16, pp. 45-51 (1974); and Houlihan et al., Mat. Res. Bull., Vol. 11, pp. 1191-1198, (1976). None of these articles disclose any effect of the anode surface of an n-type TiO.sub.2 semiconductor on O.sub.2 yield for absorbed photons, quantum efficiency of the cell for absorbed photons, and surface reflection loss. In addition, these references are silent on how to optimize the current available at the cathode through the use of a titanium diffusion junction ohmic contact.
Hardee et al. indicate that prior uses of TiO.sub.2 anodes taught the preparation of a uniform, shiny, blue-gray TiO.sub.2 coating on a Ti substrate by chemical vapor deposition (CVD). U.S. Pat. No. 3,271,198, to Winogradoff et al. teaches etching a single crystalline surface to produce a smooth surface to reduce surface recombination losses caused by surface roughness, irregularities and impurities. Ohnishi et al. disclose the use of single crystals of TiO.sub.2 in the form of wafers, 10.multidot.10.multidot.1 mm, with optically flat (001) surfaces, i.e., smooth with respect to the incident light.
Initially, mirror finished TiO.sub.2 semiconductor anodes, as outlined in numerous articles, were utilized to generate oxygen. However, measurements of oxygen gas evolution and photocurrent indicated that the smooth finished n-type semiconductor TiO.sub.2 anodes did not provide sufficient combination centers for the holes generated by solar radiation to combine with hydroxyl ions (OH.sup.-) and generate O.sub.2. Abrasively roughened surfaces provided an overabundance of recombination centers with the resultant further lowering of the O.sub.2 evolution and much lower photo-current than the theoretically calculated quantum yield of current. The lower the photocurrent is from the theoretically calculated quantum yield, the lower also will be the yield of hydrogen gas from the cathode.
With respect to lower photocurrent, indium ohmic contacts, known in the art, provide only about 66% of the reaction current of titanium diffusion junction ohmic contacts formed by electron beam evaporation of titanium on the n-type TiO.sub.2 semiconductor anode.
Our earlier filed application Ser. No. 760,551, filed Jan. 19, 1977 now U.S. Pat. No. 4,061,555, incorporated herein by reference, teaches an improved nickel-nickel monoxide cathode wherein the cathode is grooved to expose the nickel substrate below the nickel monoxide insulating layer and provide high density catalytically active centers for hydrogen evolution. The grooves in the cathode were made to restrict the current flow to the groove regions and thus to increase the circuit impedance in the grooved region to a value about that of the depletion layer at the n-type semiconductor anode, thereby allowing the water photolysis reaction to proceed with little or no applied biased voltage.